I. Field of the Invention
The present invention relates to a method for forming a conductor pattern of a semiconductor device, and more specifically to a method for forming a gate electrode of an MESFET.
II. Description of the Prior Art
The following is an example of a prior art method for forming a conductor pattern on a semiconductor substrate. First, as shown in FIG. 1A, a metal layer 2 is formed on a semiconductor substrate 1. Then, as shown in FIG. 1B, a photoresist pattern 3 is formed on target region 2a of the metal layer 2 to carry an electrode pattern thereon. Thereafter, as shown in FIG. 1C, the metal layer 2 is subjected to reactive ion etching using the photoresist pattern 3 as a mask, thus forming a metal electrode pattern 2a. Finally, as shown in FIG. 1D, the photoresist pattern 3 is removed to leave the metal electrode pattern 2a alone on the semiconductor substrate 1.
According to this conventional method, however, the use of the photoresist pattern as the etching mask involves the following problems.
(1) The photoresist pattern 3 is not very resistant to reactive ion etching and needs to be made relatively thick. As the photoresist pattern 3 is increased in thickness, the dimensional change becomes proportionally greater. Even if the width of the photoresist pattern 3 is e.g. about 0.5 microns, the electrode pattern 2a of the metal layer 2 will sometimes spread out at the skirt so that the pattern width m at its junction with the semiconductor substrate 1 will be about 1.0 micron. When forming an electrode pattern of a thick metal layer, patterning is difficult since the etching time is so long that the photoresist is removed by etching before the electrode pattern is formed.
(2) As shown in FIGS. 2B and 2C, the photoresist pattern 3 is subjected to side etching in the directions indicated by the arrows, so that the etching effect reaches the lateral faces of the electrode pattern 2a of the metal layer, rendering the vertical section of the electrode pattern 2a trapezoidal (FIG. 2B) or semicircular (FIG. 2C) in shape. The electrode pattern subjected to such side etching would acquire higher resistivity than that of the rectangular electrode pattern. High resistivity may cause noise and poorer high-frequency characteristics.
(3) In the manufacture of a semiconductor device, processes are sometimes required after the formation of the electrode pattern 2a in which a mask film 5 is formed on the lateral faces of the electrode pattern 2a (side wall process), and the semiconductor substrate 1 is subjected to ion implantation as indicated by arrows 6 using the mask film 5 and the electrode pattern 2a as a mask (self-alignment process), as shown in FIG. 2D. If the vertical section of the electrode pattern 2a is trapezoidal or semicircular as mentioned, it is impossible to accomplish accurate self-aligning ion implantation.
(4) It is difficult to form a fine photoresist pattern with a thickness of e.g. 0.5 microns or less, since it may sometimes be dissolved away by a developing agent during the developing process, as shown in FIG. 2E. The photoresist is very resistant to etching, and even if patterned with success, it could possibly peel off during an etching process in which it is used as a mask for forming a metal electrode pattern. It is therefore very hard to form a fine metal electrode pattern using the photoresist as a mask.
The following is another prior art method forming a conductor pattern on a semiconductor substrate.
First, as shown in FIG. 3A, a gold layer 12 to form an electrode pattern and a titanium layer 13 to serve as an auxiliary mask are successively formed on a semiconductor substrate 11. Thereafter, a photoresist pattern 14 corresponding in shape to the electrode pattern to be formed is formed on the titanium layer 13.
Then, as shown in FIG. 3B, the titanium layer 13 and the gold layer 12 are selectively removed by the ion milling method using the photoresist pattern 14 as a mask. In doing this, accelerated argon particles 15 are impinged on the surface of the structure to selectively remove the titanium layer 13 first and then the gold layer 12. In removing the gold layer 12, the photoresist pattern 14 and the remaining titanium layer 13 are used as a mask. Thus, a gold electrode pattern 12a is formed.
Also in this method, the photoresist is used as a mask, so that it is impossible to form an accurate mask pattern. Accordingly, it is difficult to form a fine conductor pattern, and the dimensional change is great. When forming the electrode pattern by selectively removing the gold layer using the ion milling method, moreover, titanium or gold powder will stick to the lateral faces of the thick photoresist pattern increasing its width. If the photoresist pattern is widened in this manner, the electrode pattern is also widened, making it impossible to form a fine electrode pattern.